Internet, I need your help!

I have spent almost an entire day of my vacation playing with the damn computer. Not an entirely bad thing, because I think it is somewhere around 57 degrees centrigrade outside, and I am huddled in the relatively cool basement with my sexy husband. (I am dictating, he is typing. Could you tell?)

Ahem (wrenching control of the keyboard) - as I was saying... We've spent the better part of the day playing with Photoshop, and have come up with a relatively cute little banner for blog. But, I cannot for the life of me figure out how to post it. I've played in the CSS of the existing template a bit, but honestly thought it would be as easy as an IMG tag. Apparently not. I've studied some of the CSS on blogs in my Blogroll, but don't quite get it.

Is there an easy answer for this? I just want to use the existing template for now and instead of the header at the top, I want my banner to appear. I can host the image on server space from my cable Internet provider, but when I just key in the image tag with that addy, nothing happens. Do I have to build a little CSS script to define the banner space and centre it and point to the image or something? Also, if you are giving free advice, how do I include images in between my posts and/or change my sidebar headers to images I will design in some future year when the boys go off to college.

If I need to teach myself more about CSS I will, but I'd prefer a quick fix!